This invention relates generally to a plastic injection manifold for dividing an incoming stream of molten material, which material is thermally degradable plastic, into a plurality of smaller streams for delivery to several individual molds. More specifically, this invention relates to a manifold structure so designed as to maintain a constant velocity of all the material being handled at a particular time whereby the temperature thereof can be more closely controlled than has been possible heretofore.
The increasing use of plastic materials, especially in the container industry, has created a need for a moldable material capable of withstanding the extreme environmental conditions to which containers of glass and other materials have been subjected. Efforts to develop such materials have led to the introduction of materials which are characterized by higher melting points than have been encountered heretofore. For example, molten polyvinylchloride (PVC) can be handled in prior art manifolds at temperatures of less than 400.degree.F if the pressure in the manifold is kept sufficiently high. This material degrades at approximately 500.degree.F, so that the PVC manifold need only keep the molten material at 400.degree., .+-.50.degree.F in order to provide for the efficient handling of the PVC in such manifolds and also to prevent it from degrading.
Newer plastics have been developed, especially for the container industry, from acrylics and nitriles, which materials still degrade at approximately 500.degree.F, but which can only be caused to flow at reasonable pressures at temperatures on the order of 450.degree.F or higher. Thus, a manifold structure suitable for handling such materials must be capable of maintaining all of the material in such manifold at a temperature which is necessarily closely controlled. For example, the temperature of this material must be held between narrow limits, such as 475.degree., .+-.10.degree.F. The chief aim of the present invention is to provide a manifold structure which will permit maintaining this degree of control over the temperature of all of the material being handled in the manifold.
Prior art manifolds have been designed to handle plastics, such as PVC, by increasing the pressure so that the material can be forced through the manifold's passageways at lower temperatures. This approach has obvious disadvantages in the handling of PVC, and becomes impractical in the handling of the more critical acrylics and nitriles.
The general aim of the present invention is to provide a manifold so designed that all of the plastic material is moved through all of its passageways at a velocity which is kept constant throughout the manifold at least at any one time during the cycle of operation of the injection molding machine. Sharp corners and any protuberances in these passageways are carefully avoided to decrease friction, and the cross sectional area of each passageway or branch is progressively reduced between the inlet and the plurality of outlet nozzles to keep the instantaneous speed of every particle of material in the manifold at a level which is the same as every other particle.